The present invention relates to a DC-to-DC converter and more particularly to a DC-to-DC converter with synchronous rectifiers utilizing a combined transformer-inductor device.
In semiconductor industry, the manufacturing process migrates to finer pitch with time for higher degree of integration and hence to better electrical performance. At lower pitch (i.e. equal to or less than 0.25 micrometer), the operating voltage of an IC (integrated circuit) decreases from 2.5V, 1.8V, and 1.5V to sub one volt. When more and more transistors are integrated into an IC, higher operating current is required. When a switching power converter delivers energy to drive ICs, the high conduction in rectifiers and long trace length often lead to low power conversion efficiency. The physical performance limitation factor of a power converter is the power density defined in Watts per cubic inch or Amperes per cubic inch. This invention improves the efficiency of power converters and reduces the size at a certain power density.
An approach to improve the efficiency of power converter for low voltage and high current output is to employ synchronous rectification. Synchronous rectification refers to using active devices such as MOSFET as a replacement for Schottky diodes for rectification in circuit. Since the drop voltage of the synchronous rectifier is much less than the drop voltage of diodes, the conduction loss of rectifier can be reduced greatly.
Another approach to reduce the conduction loss and improve the power density of the converter is disclosed by U.S. Pat. No. 4,803,609, issued to John B. Gillett, for xe2x80x9cD.C. to D.C. Converterxe2x80x9d, which combines transformer device and output filter inductor to reduce the size of the energy storage components and minimize the conduction loss of secondary winding and connection trace.
Another approach to improve the efficiency of power converters is to maximize the energy of the switching devices and the transformer during each switching cycle in order to improve the power transfer characteristic of the converter as indicated in U.S. Pat. No. 4,441,146 issued to P. Vinciarelli for xe2x80x9cOptimal resetting of the transformer""s core in single ended forward convertersxe2x80x9d. This approach is to use a clamped mode topology to reduce the power loss of the primary switch and recycle the leakage energy of the transformer. And in U.S. Pat. No. 5,303,138 issued to Allen F. Rozman for xe2x80x9cLow loss synchronous rectifier for application to clamped-mode power convertersxe2x80x9d the combination of this clamped mode topology and synchronous rectifier is disclosed, thereby achieving higher conversion efficiency.
However, each of the above disclosures indicates some facts of the power conversion issue, none provides an approach achieving all of the advantages discussed above. Furthermore, with the increase of switching frequency, the core loss of transformer is also a significant factor in power loss dissipation. For the purpose of high efficiency and high power density, the primary switch loss, synchronous rectifier loss, transformer winding loss and transformer core loss all should be minimized.
For the above reasons, a need still exists in the art of designing and manufacturing DC/DC converter to provide an optimal configuration for low voltage and high current output. The improved converter configuration will be described in this invention as below.
It is therefore an object of the present invention to propose a high power density and high efficiency DC-to-DC converter for generating a low voltage output and a high current output by utilizing a combined transformer-inductor device and a synchronous rectification circuit.
It is therefore another object of the present invention to propose a clamped-mode DC-to-DC converter for generating a low voltage output and a high current output by utilizing a combined transformer-inductor device and a synchronous rectification circuit in order to minimize the primary switch loss, synchronous rectifier loss, transformer winding loss and transformer core loss.
According to an aspect of the present invention, the power converter includes an input port for accepting a DC voltage; an output port for accepting a load to be energized, a combined transformer-inductor device having a primary winding, a secondary winding, and an auxiliary winding in which the primary and secondary windings perform a transformer function and the auxiliary winding performs an output filter function; a switching circuit connected to the input port for periodically providing a positive voltage to the primary winding during a first interval of a cyclic period of the power converter and a negative voltage to reset the primary winding during a second interval other than the first interval; and a synchronous rectification circuit connected for enabling conduction from the secondary winding to the output port during the first interval and the second interval of the cyclic period, wherein the auxiliary winding is coupled between the output port and the synchronous rectification circuit for providing an inductance to filter an output signal.
Preferably, the combined transformer-inductor device further includes a magnetic core with a high reluctance flux path and a low reluctance flux path sharing a common segment. The energy from the primary winding is partly transferred to the secondary winding via the low reluctance flux path and partly stored in the high reluctance flux path during the first interval of the cyclic period.
Preferably, energy stored in the high reluctance flux path is transferred from the combined transformer-inductor device to the synchronous rectification circuit via the auxiliary winding during the second interval of the cyclic period.
Preferably, an air gap is positioned in the common segment to reduce an output current ripple.
Preferably, the switching circuit further includes a clamp device for limiting a voltage across the transformer during the second interval of the cyclic period.
Preferably, the synchronous rectification circuit includes a first synchronous rectification device connected for enabling conduction from the secondary winding to the output port during the first interval of the cyclic period, and a second synchronous rectification device connected for enabling conduction from the secondary winding to the output port during the second interval of the cyclic period.
Preferably, the synchronous rectification circuit includes a first synchronous rectification device with a control gate responsive to a voltage of the secondary winding and biased periodically conducting and connected for conducting current from the secondary winding to the output port, and a second synchronous rectification device connected for conducting from the secondary winding to the output port when the first synchronous rectification device is non-conducting.
Preferably, the first synchronous rectification device is a synchronous rectifier.
Preferably, the second synchronous rectification device is a synchronous rectifier.
Preferably, the synchronous rectifier is a MOSFET.
It is therefore another aspect of the present invention to propose a power converter including an input port for accepting a DC voltage; an output port for accepting a load to be energized; a combined transformer-inductor device having a primary winding, a secondary winding, and an auxiliary winding in which the primary and the secondary windings perform a transformer function and the auxiliary winding performs an output filter function; a switching circuit connected to the input port for periodically providing a positive voltage to the primary winding during a first interval of a cyclic period of the power converter and a negative voltage to reset the primary winding during a second interval other than the first interval, wherein the switching circuit further includes a clamp device for limiting a voltage across the transformer during the second interval of the cyclic period; and a synchronous rectification circuit connected for enabling conduction from the secondary winding to the output port during the first interval and the second interval of the cyclic period, wherein the auxiliary winding is coupled between the output port and the synchronous rectification circuit for providing an inductance to filter an output signal.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: